Container assembly having coupling lugs

ABSTRACT

A container assembly for use as a sampling device for microorganisms includes a base member and a lid removably coupled to the base member. The lid includes a top wall and a side wall depending from the top wall. The base member has a side wall with an outer surface that complements the inner surface of the side wall of the lid. A plurality of lugs are formed on the outer surface of the base member to provide a gripping force to secure the lid to the base member. The gripping force is provided by the surface area of the lugs so that the lid is secured to the base member while enabling the lid to be readily removed from the base member without sticking or binding. The lugs are odd in number, are uniformly spaced around the side wall of the base member, and are dimensioned to provide a point of contact with the outer wall of the lid causing the wall of the lid to flex outwardly at the point of contact so that the distortion and flexing of the wall of the lid provide a resilient gripping force against the lugs of the base member. The lugs are spaced apart a distance so that the wall of the lid is able to flex inwardly between adjacent points of contact between the lugs and the wall of the lid when the lid is distorted. In one embodiment, three lugs are provided and uniformly spaced apart so that no two lugs are diametrically opposed from one another and a point of contact and outward deflection of the wall of the lid is diametrically opposed to an area of inward deflection of the wall of the lid.

FIELD OF THE INVENTION

The present invention is directed to a container assembly for use in the sampling and growth of microorganisms. More particularly, the invention is directed to a container assembly having a base and a lid where the lid is removably coupled to the base by lugs on the base.

BACKGROUND OF THE INVENTION

Containers for culturing microorganisms are commonly referred to as petri dishes and are well known in the art. These devices typically include a bottom container or base for the growth medium and a peripheral side wall. The bottom container is typically used in conjunction with a lid having a top wall and a peripheral side wall that is able to fit over the side wall of the bottom container. Generally, the side wall of the lid forms a loose fit with the bottom container to prevent the lid from binding with the bottom container. Containers such as these are manufactured by several manufacturers and are readily available.

Sample containers and petri dishes for microorganisms are commonly used in large numbers each day by a technician. A technician can use several hundred containers each day to test for various microorganisms. To enable the technician to handle the large number of containers efficiently, the lids must be easily removed and replaced without sticking or binding together. In some instances, it is desirable to have the lid sufficiently loose to enable the technician to remove the lid with one hand while dispensing the sample into the petri dish for testing. Other uses of the petri dish require that the lid be secured to the dish to prevent inadvertent separation of the lid from the bottom container but still allow the lid to be removed from the dish with minimal effort.

The petri dishes which have been inoculated with a sample are often transported in a stack. The petri dishes often have some form of restraining structure to prevent sliding of the dishes with respect to each other when they are stacked. The dishes generally have a recess in the bottom portion that can mate with the lid of another petri dish when stacked.

One example of a petri dish having a container and a lid is disclosed in U.S. Pat. No. 5,021,351. The petri dish as disclosed in this patent includes a bottom container with an annular base that is smaller than the outermost dimension of the base. The lid includes a recessed portion that can receive the annular ring of the base to prevent sliding of the petri dishes when stacked. The petri dishes are stacked by nesting the bottom of the base in the recess of the lid.

One problem that often occurs with petri dishes during use is the risk of the technician touching the agar media or other growth media. This typically results in inaccurate sampling results since unwanted microorganisms are often transferred to the growth media by the inadvertent touching. To reduce the risk of inadvertent contact with the growth media, various methods have been proposed for securing the lid to the base container to prevent inadvertent separation of the lid from the base container. These efforts have often resulted in high frictional forces that can make it difficult to separate the lid from the container. Other methods have been generally ineffective or difficult to operate. One example of such a device is disclosed in U.S. Pat. No. 5,725,123. This patent discloses a petri dish having a lid with several elevations that contact the side wall of the base container to form a secure friction fit. The sides of the lid are compressed to allow the elevations to be released from the base. This arrangement has the disadvantage of requiring the technician to grip the lid in specific locations to separate the lid from the base.

Examples of other petri dishes are disclosed in U.S. Pat. No. 6,602,704, U.S. Pat. No. 5,854,065, U.S. Pat. No. 3,769,936 and U.S. Pat. No. 4,945,061. These patents generally disclose a base container having a lid that closes the open top end of the base container.

The prior petri dishes have been generally successful for their intended purpose. However, there is a continuing need in the industry for improved containers for culturing microorganisms.

SUMMARY OF THE INVENTION

The present invention is directed to a container assembly for use in sampling microorganisms. The invention is particularly directed to a container assembly having a base and a lid where the lid can be secured to the base while being readily separable from the base.

Accordingly, an object of the invention is to provide a container assembly having a base and a removable lid. The container base has a peripheral edge with a plurality of lugs that contact the lid when the lid is placed on the base. The lugs define a friction fit to resist inadvertent separation of the lid from the base while allowing the lid to be separated easily by the technician during use.

In one embodiment, the present invention is directed to the container assembly having a base and a lid that is removably coupled to the base where the lid grips the base with sufficient frictional force to enable the container assembly filled with a growth medium to be lifted by the lid without separation of the base.

Another object of the invention is to provide a container assembly with a continuous side wall where the side wall includes a plurality of lugs protruding outwardly from the outer surface of the side wall. The lugs are positioned to engage the side wall of the lid with sufficient friction to couple the lid to the base.

The invention primarily relates to a device for use in connection with sampling of a surface for the presence of microorganisms. The container assembly includes a base and a lid where the base has a support surface for receiving a growth media such as an agar media. A side wall surrounds the support surface and depends from the peripheral edge of the support surface. The side wall of the base includes a flange that extends radially outward to form a lid support surface. A lip depends from the outer peripheral edge of the lid support surface and extends in a generally downward direction with respect to the lid support surface and the media support surface.

The side wall of the base includes a plurality of lugs that protrude outwardly from the surface of the side wall. The side wall preferably includes an odd number of lugs which are uniformly spaced apart with no two lugs being diametrically opposed. The lugs have a shape and dimension to engage the inner surface of the lid and frictionally grip the inner surface of the lid.

The various objects of the invention are obtained by providing a container assembly comprising a base member and a lid, where the base member has a support surface and a continuous side wall surrounding the support surface and has a longitudinal dimension extending in a direction away from the support surface. The side wall of the base member has a plurality of lugs protruding outwardly from an outer surface of the continuous side wall. The lid has a substantially planar surface and a continuous side wall depending from the planar surface. The side wall of the lid has an inner surface and an inner dimension complementing an outer dimension of the continuous side wall of the base member. The lugs on the continuous side wall of the base member are spaced apart in such a manner that the side wall of the lid deflects outwardly in a contact area of the lugs and the side wall deflects inwardly in an area between adjacent lugs on the side wall of the base member.

The objects of the invention are also obtained by providing a container assembly for use in sampling the surface for the presence of microorganisms comprising a base and a lid. The base has a support surface for supporting a sample media and a continuous side wall surrounding the support surface and having a longitudinal dimension extending away from the support surface. The outer surface of the side wall of the base has an odd number of lugs protruding outwardly from the side wall. A peripheral lid support surface extends outwardly from the continuous side wall of the base. The lid has a substantially planar surface and a continuous side wall extending from the planar surface and having an inner surface and an inner dimension. The lid is dimension to be placed on the base and overlie the side wall of the base. The lugs on the side wall of the base have a dimension to contact the inner surface of the side wall of the lid and are spaced apart a distance to deflect the side wall of the lid outwardly at a point of contact with the lugs and to deflect the side wall of the lid inwardly at a point between adjacent lugs on the base.

The various features, objects and advantages of the invention will become apparent to those skilled in the art in view of the following detailed description and the annexed drawings which disclose preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings in which:

FIG. 1 is a side elevational exploded view of the container assembly according to one embodiment of the invention showing the base and lid;

FIG. 2 is a top view of the base of FIG. 1;

FIG. 3 is a bottom view of the base member of the embodiment of FIG. 1;

FIG. 4 is a bottom view of the lid of the embodiment of FIG. 1;

FIG. 5 is a partial enlarged top view of the base showing a lug in one embodiment of the invention;

FIG. 6 is a top view of the container assembly depicting the flexing of the lid when placed on the base;

FIG. 7 is a partial cross-sectional side view of the base and lid showing the side wall and the lug on the base;

FIG. 8 is a perspective view of the base showing the lug on the side wall of the base;

FIG. 9 is a partial cross-sectional view taken along line 9-9 of FIG. 6 showing the lid fitted on the base and showing the lug contacting the lid and deflecting outwardly from the base;

FIG. 10 is a partial cross-sectional view taken along line 10-10 of FIG. 6 and showing the lid fitted on the base and deflecting inwardly in an area between adjacent lugs from the coupling engagement of the lid to the base;

FIG. 11 is a top view of a base in a second embodiment of the invention; and

FIG. 12 is a side view in cross-section showing the dimensions of the lugs in one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary embodiment of the present invention is a container assembly 10 having a base member 12 and a lid 14. Container assembly 10 has a dimension and shape suitable for supporting growth media for growing microorganisms, such as bacteria. In preferred embodiments of the invention, container assembly 10 has a width and a height as commonly used in the industry for devices used in sampling microorganisms.

Referring to the drawings, container assembly 10 typically has a substantially circular shape as commonly used in the industry. The base 14 has a bottom wall 16 that forms a support surface for growth media for the microorganisms. In the embodiment illustrated, bottom wall 16 has a slightly convex upper surface 18 and a slightly concave bottom surface 20. Preferably, bottom wall 16 is a support surface suitable for supporting a conventional agar growth medium which can be used for sampling surfaces to determine the presence or absence of microorganisms. In one embodiment, bottom wall 16 includes a plurality of perpendicular intersecting ridges 22 to define a grid system 24. Grid system 24 is provided to facilitate counting of microorganisms as known in the art.

Base member 12 includes a side wall 26 that extends away from the plane of bottom wall 26 in a generally upward axial direction with respect to an axial dimension of base member 12 and to define a height of side wall 26. Side wall 26 is a continuous structure that encircles bottom wall 16 and defines an upper cavity 28 in base member 12. Preferably, side wall 26 has an annular or circular shape that surrounds the peripheral edge of bottom wall 16 to contain growth medium and receive lid 14. Side wall 26 has a cross-section as shown in FIG. 7 with a substantially inverted U-shape formed by an inner wall 30 that extends upward from the peripheral edge of bottom wall 16, a top portion 32 and an outer wall 34 that extends in a generally downward direction from top portion 32. In the embodiment illustrated, inner wall 30 and outer wall 34 are oriented in substantially the same direction and have a slight taper that converge toward top portion 32 to assist in manufacturing and facilitating removal of the base member 12 from the mold used to form base member 12. Base member 12 is typically made as a unitary, single piece from a suitable transparent plastic material such as polystyrene. Typically, base member 12 is made by injection molding so that the taper of inner wall 30 and outer wall 34 facilitate removal of the injection molded part from the mold cavity.

Referring to FIGS. 7-10, outer wall 34 has a distal bottom end and a flange 36 extending therefrom. Flange 36 in the embodiment illustrated extends in an outward radial direction with respect to base member 12. Flange 36 in preferred embodiments is continuous and has a substantially annular shape and defines a lid supporting surface with a shape and dimension for receiving and supporting lid 14. Preferably, flange 36 has a uniform width surrounding outer wall 34 and lies in a horizontal plane that is substantially perpendicular to a vertical axis of base 12.

As shown in FIG. 7, a continuous wall 38 depends from a peripheral edge of flange 36 in a downward direction. As shown, wall 38 extends away from bottom wall 16 and flange 36 in a generally downward direction and defines a circular supporting wall for base member 12. Wall 38 includes an upper edge 40 that is contiguous with flange 36 and extends to a free bottom edge 42. Preferably, wall 38 has a substantially annular shape to stabilize base member 12 when placed on a table or other support surface. In a preferred embodiment, the outer diameter of bottom edge 42 of wall 38 is greater than the diameter of upper edge 40 to assist in stabilizing base member 12 and to assist in stacking and nesting of multiple container assemblies.

Base member 12 has a bottom cavity 44 defined by bottom wall 16 and wall 38 as shown in FIGS. 7, 9 and 10. A plurality of ribs 46 as shown in FIG. 10 extend in a generally inward direction from an inner surface of wall 38. Each rib 46 has a bottom surface 48 that is contoured to received an upper portion of a lid 14 when multiple container assemblies are stacked. Preferably, bottom surface 48 of ribs 36 have a shape and dimension to contact an upper surface of lid 14. In one embodiment, four ribs 46 are provided and uniformly spaced apart about 90° within bottom cavity 44. The fit between a lid 14 and ribs 46 on an adjacent base member 12 preferably provide a reliable stacking arrangement to resist the stacked container assemblies from sliding laterally with respect to each other. Preferably, ribs 46 and wall 38 have a dimension to receive an upper portion of a lid 14 without binding or sticking so that a container assembly 10 can be lifted from a stack without inadvertent separation of a lid 14 from a respective base member 12.

Lid 14 has a generally planer top surface 50 as shown in FIG. 1. Lid 14 is preferably made of a transparent plastic material such as polystyrene that is rigid but is able to provide some limited flex to lid 12 without cracking or breaking. In the embodiment shown, top surface 50 has a slightly convex shape that is referred to herein as being generally planar. In alternative embodiments, top surface 50 can have a slightly concave shape as desired. Lid 14 has a side wall 52 depending from a peripheral outer edge 54 of top wall 50 of lid 14. Side wall 52 extends in a generally downward direction with respect to top wall 50 and has a slightly outward taper. Referring to FIG. 7, side wall 52 has a distal end 56 having an outer dimension that is slightly greater than the outer dimension of top wall 50 so that side wall 52 has a slightly frustoconical shape. In one embodiment of the invention, side wall 52 has a taper of about 7° with respect to a vertical axis of lid 14.

Distal end 56 of side wall 52 of lid 14 has a flange 58 that extends radially outward from side wall 52 as shown in FIGS. 1 and 7. Flange 58 has a substantially annular shape with a uniform width. Flange 58 has an outer diameter that is greater than the outer diameter of distal end 56 of side wall 52. Flange 58 provides a visual and tactile indication to the user of the position of lid 14 to reduce the risk of the user inadvertently contacting bottom wall 16 of base member 12 and contaminating the growth media on bottom wall 16.

Flange 58 has a bottom surface 60 that faces in a generally downward direction with respect to the normal orientation of lid 14 when placed on base 12. As shown in FIG. 4 and FIG. 7, a plurality of protrusions 62 shown as bumps that extend outwardly from bottom surface 60. In a preferred embodiment, protrusions 62 are uniformly spaced apart around the periphery of bottom surface 60 of flange 58. Protrusions 62 have a shape and dimension to space bottom surface 60 of flange 58 from flange 36 of base member 12.

Flange 36 of base member 12 supports side wall 52 of lid 14 when lid 14 is placed on base member 12 as shown in FIG. 10. Protrusions 62 contact flange 36 to form a small gap between flange 36 of base member 12 and flange 58 of lid 14. The small gap allows for air to flow into and out of cavity 28 as may be desired or necessary for many applications in promoting proper growth of microorganisms. The gap formed between flange 36 and flange 58 also prevent a moisture lock or seal from being inadvertently created between lid 14 and base 12 so that lid 14 can be readily separated from base 12.

Referring to FIGS. 5 and 6, the outer surface of outer wall 34 includes a plurality of lugs 70. In one preferred embodiment of the invention, lugs 70 are integral with the outer surface of outer wall 34 and flange 36 as shown. Lugs 70 have a shape and dimension to contact an inner surface of side wall 52 of lid 14 to provide a secure friction fit engagement between lid 14 and base 12 while allowing lid 14 to be easily separated from base 12 by the technician. In preferred embodiments of the invention, lugs 70 are uniformly spaced apart around outer wall 34. Lugs 70 are positioned to engage the inner surface of side wall 52 of lid 14 to form a point of contact with side wall 52 and apply a substantially uniform outward gripping force. Typically, lid 14 is made of a transparent plastic material, such as polystyrene. Polystyrene is slightly flexible but brittle and can easily crack or break. Lugs 70 have a radial dimension and a radius of curvature to be able to flex side wall 52 of lid 14 at a point of contact between lid 14 and lug 70 in a substantially radially outward direction with respect to lid 14 as indicated by arrows 76 in FIGS. 6 and 9 without cracking or breaking. The resilience and flexibility of side wall 52 of lid 14 form a gripping force against lugs 70 and define a friction fit. Lugs 70 are spaced apart a sufficient distance so that side wall 52 of lid 14 is deflected inwardly in an area between adjacent points of contact of adjacent lugs indicated by arrows 78 in FIGS. 6 and 10.

Lugs 70 are oriented and spaced apart a distance to provide a limited and controlled distortion of side wall 52 of lid 14 when lid 14 is placed on base member 12. Preferably, base 12 and lugs 70 are sufficiently rigid so that side wall 26 of base 12 is dimensionally stable and does not flex or distort when lid 14 is placed on base 12. In preferred embodiments of the invention, side wall 52 of lid 14 has a substantially annular shape which can be distorted outwardly at the point of contact with lugs 70 as indicated by arrows 76 and distorted or deflected inwardly in the areas between the point of contact of adjacent lugs as indicated by arrows 78 as shown in FIGS. 6, 9 and 10. When lid 14 is placed on base member 12, side wall 52 of lid 14 assumes a slightly distorted, non-circular shape as a result of the relative flexibility of lid 14 and the relatively inflexible base member 12 which is typically manufactured with walls slightly thicker than those of the lid 14. To achieve the proper distortion of side wall 52 of lid 14 and the desired gripping force, the number, dimensions and spacing of lugs 70 is important. In the embodiment of FIGS. 5 and 6, the surface of outer wall 34 of base member 12 includes three lugs 70 that are uniformly spaced apart. In this manner, no two lugs are diametrically opposed to each other on outer wall 34. In addition, the uniform spacing of lugs 70 form an area for inward deflection of side wall 52 of lid 14 that is diametrically opposed to a point of contact between a respective lug 70 and side wall 52. The space between adjacent lugs 70 on base 12 define the area for inward deflection for side wall 52 of lid 14 while lugs 70 define an area for outward deflection of side wall 52 of lid 14. Preferably, side wall 26 and lugs 70 are sufficiently rigid to prevent bending or flexing of base 12 and cause lid 14 to flex or distort into a non-circular shape when lid 14 is placed on base 12.

In another embodiment of the invention, five lugs 70 are provided on outer wall 34 and are uniformly spaced apart. It has been found that five lugs 70 can provide the proper spacing between the points of contact between lugs 70 and side wall 52 to provide the desired outward deflection at the point of contact and the desired inward deflection or deformation between adjacent points of contact. It has also been found that more than five lugs on a standard size container assembly provide a radial surface area that does not enable proper bending or deflection of side wall 52 of lid 14 and can cause cracking or breaking of side wall 52 of lid 14. When the surface area of the lugs becomes too great, the lugs cause the entire side wall of the lid to expand radially outward rather than flexing and distorting which can cause the lid to crack or result in excessive frictional engagement and prevent easy separation of the lid from the base. It has also been found that fewer than three lugs on the outer wall of base 12 provide insufficient gripping and frictional forces between lid 14 and base member 12 to adequately secure lid 14 to base 12.

Lugs 70 have a dimension and shape to form a gripping effect through frictional forces between the contact surface of a respective lug 70 and the inner surface of side wall 52 of lid 14. Preferably, the frictional forces are sufficient to secure lid 14 on base member 12 without cracking or breaking lid 12 and while allowing the removal of lid 12 by the technician without excessive sticking or binding. In a preferred embodiment of the invention, the frictional forces between lugs 70 and side wall 52 of lid 14 enable the technician to remove lid 14 from a respective base member 12 with one hand during the handling of container assembly 10 and processing of the microorganisms. In one embodiment, lid 14 is coupled to base 12 by sufficient frictional forces to enable assembly 10 to be lifted by lid 14 without inadvertent separation of base member 12 from lid 14. Preferably, the gripping force is in terms of a gram-weight-fit so that assembly 10 can be lifted by lid 14. In one embodiment, the friction gripping force is about twice the weight of base member 14. In other embodiments, the gripping force by the lugs engaging the inner surface of the side wall of the lid is about 2 to about 20 times the weight of the base member.

In one embodiment of the invention, base member 12 and lid 14 are made of transparent polystyrene, which is typically brittle and can easily crack. Lugs 70 have a radius and a dimension to provide a rounded outer face that forms a gradual or rounded bending surface of outer wall 52 of lid 14 to resist cracking or breaking of lid 14. In addition, the rounded surfaces and dimension of lugs 70 also inhibit cracking or chipping of lugs 70 during continuous use. It has been found that a sharp angle on the outer surface of lugs 70 can cause lid 14 to crack and can result in lugs 70 chipping or cracking.

Referring to FIG. 5, lugs 70 have a height extending in an axial, longitudinal direction with respect to outer wall 34 of base member 12 that provide a surface area sufficient to contact and grip side wall 52 of lid 14. In one embodiment as shown in FIG. 5, lugs 70 have a height that is about one-half the height of outer wall 34. The height of lugs 70 can vary to provide a surface area with the necessary friction gripping force for lid 14. Preferably, each of the lugs 70 have substantially the same dimensions and shape so that each lug contacts the inner surface of side wall 52 of lid 14 with a uniform gripping force.

Referring to FIG. 6, lugs 70 in a preferred embodiment of the invention have a substantially arcuate and cylindrical outer side 72 forming a substantially convex surface. In a preferred embodiment, outer side 72 has a substantially continuous and uniform shape. Lugs 70 have an upper portion 74 having a substantially semi-spherical shape that converges from outer side 72 and is contiguous with outer wall 34 of base member 12. Upper portion 74 has a shape and dimension to guide side wall 52 of lid 14 over lugs 70. Preferably, outer side 72 extends to and is contiguous with flange 36. In one embodiment of the invention, outer side 72 of lugs 70 have an arcuate shape defined by a radius of about 0.125 to about 0.14 inch. In one preferred embodiment, outer side 72 of lugs 70 have an arcuate shape defined by a radius of about 0.125 inch to about 0.130 inch.

It has been found that lugs having a substantially arcuate shape side provide a sufficient surface area to contact the inner surface of side wall 52 of lid 14 to provide the desired outward deflection of side wall 54 at the point of contact without cracking or breaking side wall 54. Lugs that are formed with a sharp point or ridge can cause side wall 52 of lid 14 to bend around the sharp corner which can cause side wall 52 to crack and provide insufficient surface area at the point of contact to secure the lid to the base member with the desired frictional forces.

The dimensions and the contact surface area of lugs 70 are selected to provide the desired and substantially uniform gripping force for lid 14. The actual dimensions of lugs 70 will vary depending on the dimensions of lid 14 and the flexibility of lid 14 and particularly side wall 52 of lid 14. The height and thickness of side wall 52 of lid 14 determine the flexibility of the lid and the gripping force applied to the lugs 70. The dimensions and thickness of flange 58 also influences the flexibility of side wall 52. The dimensions of lugs 70 are preferably selected according to the flexibility of lid 14, and particularly side wall 54 of lid 14. For example, the dimensions and particularly the radial dimension of lugs 70 are increased to complement a lid 14 having a thin and flexible side wall 54. A lid with a thin and very flexible side wall will easily flex and distort to a non-circular shape and may not provide the desired gripping force. In this situation, the radial dimension of the lugs 70 are increased to increase the gripping force. In a similar manner, a more rigid side wall 54 of a lid 14 will not flex as easily and can cause the gripping force to be too high such that the lid is not easily removed from the base member. In the embodiments where the side wall 54 of lid 14 is less flexible, the radial dimension of lugs 70 are decreased to prevent an overtight fit and reduce the risk of cracking or breaking the lid while ensuring the desired friction fit.

In one embodiment of the invention, side wall 34 of base member 12 and side wall 52 of lid 14 are formed such that the respective side walls are substantially parallel when lid 14 is placed on base member 12. In one embodiment, side wall 34 and side wall 52 are oriented to provide a taper of about 7° with respect to a vertical axis. Outer sides 72 of lugs 70 are preferably oriented at an angle that is sufficient to grip the inner surface of side wall 52 of lid 14 without binding or sticking. An angle of outer surface 72 that is too shallow can prevent side wall 52 of lid 14 from properly engaging the respective lug such that the flexibility of side wall 52 can cause side wall and lid 14 to slide upwardly along outer side 72 of lug 70. In one preferred embodiment of the invention, lugs 70 are provided with side wall 72 having an angle that is slightly less than the angle of side walls 52 of lid 14. In the embodiment illustrated, outer side 72 of lug 70 is oriented at a taper of about 4° with respect to a vertical axis of base 12 and has been found to effectively grip lid 14 without binding or sticking when side wall 52 of lid 14 has about a 7° taper. In this embodiment, lugs 70 have a bottom section defined generally by outer side 72 and a top section at adjacent upper portion 74. The bottom section of outer side 72 has a radial dimension with respect to base 12 that is greater than the radial dimension of the top section of outer side 72.

Preferably, the mating surfaces of lugs 70 and side wall 52 of lid 14 have a slightly different taper to prevent binding. Mating annular surfaces that have little or no taper often bind and stick together. Outer wall 34 of base member 12 and side wall 52 of lid 14 have a 7° taper in one embodiment. However, the inner surface of side wall 52 of lid 14 is spaced from outer wall 32 by lugs 70 so that side wall 52 of lid 14 typically does not directly contact outer wall 34 of base member 12. In one preferred embodiment, when lid 14 is placed on base member 14, side wall 52 of lid 14 contacts the outer gripping surfaces of lugs 70 without directly contacting outer wall 34 of base member 12. The deflection and distortion of side wall 52 of lid 14 provide the desired gripping force.

Preferably, container assembly 10 has a dimension to be compatible with other commercially available petri dishes and handling apparatus. In one preferred embodiment, base member 12 has a radius of about 2.75 inches and a height of about 0.375 inch. Outer wall 34 of side wall 26 preferably has a radius of about 2.46 inches and a height 82 of about 0.15 inch as shown in FIG. 12. In this embodiment, lugs 70 have a height 84 of about 0.075 inch and a taper of about 4°. Outer side 72 of lugs 70 preferably have a height 86 of about 0.040 inch and upper portion 74 has a height 88 of about 0.35 inch. Upper portion 74 defines a curved portion extending from the outer surface of outer wall 34 to outer side 72 of lugs 70 having a radius 90 of about 0.070 inch. Outer side 72 of lugs 70 preferably have a radius of about 0.125 inch.

The foregoing description is intended to be exemplary of a preferred embodiment of the invention. It will be understood by those skilled in the art that various changes and modifications to the disclosed embodiment can be made without departing from the purview and spirit of the invention as defined in the appended claims. 

1. A container assembly comprising: a base member having a bottom support surface, a continuous side wall surrounding said bottom support surface and extending in a direction away from said bottom support surface to define a height of said side wall, and a plurality of lugs protruding outwardly from an outer surface of said continuous side wall; and a lid having a substantially planar surface, a continuous side wall having an inner surface with an inner dimension complementing an outer dimension of said continuous side wall of said base member, said lugs on said continuous side wall of said base member being spaced apart such that said continuous side wall of said lid deflects outwardly in a contact area of said lugs and deflects inwardly in an area between adjacent lugs on said side wall of said base member.
 2. The assembly of claim 1, wherein said lugs are uniformly spaced apart.
 3. The assembly of claim 1, wherein an odd number of said lugs are provided on said outer surface of said side wall and where said lugs have a dimension to contact an inner surface of said continuous side wall and apply a substantially uniform outward gripping force to said continuous side wall of said lid.
 4. The assembly of claim 3, wherein said lugs are spaced to define an area for inward deflection of said lid between adjacent lugs of said base and where each of said lugs are diametrically opposed to an area for inward deflection of said lid.
 5. The assembly of claim 1, wherein said side wall of said base includes three of said lugs uniformly spaced apart around said base.
 6. The assembly of claim 1, wherein said side wall of said base includes five of said lugs uniformly spaced apart around said base.
 7. The assembly of claim 1, wherein said side wall of said base includes an odd number of said lugs uniformly spaced around said base and where said lugs have a longitudinal dimension, a width and a convex outer surface with a surface area sufficient to define a friction fit with said lid.
 8. The assembly of claim 7, wherein each of said lugs have a width and a radial dimension sufficient to deflect and deform said side wall of said lid to grip said lid without cracking or breaking said lid and where said continuous side wall of said lid contacts said lugs without contacting said continuous side wall of said base member when said lid is placed on said base member.
 9. The assembly of claim 1, wherein said side wall of said base and said lugs are dimensionally stable when said lid is placed on said base.
 10. The assembly of claim 1, wherein said base includes a lid support surface extending in an outward direction from said side wall of said base for supporting said side wall of said lid.
 11. The assembly of claim 10, wherein said side wall of said lid includes a flange having a plurality of spaced apart protrusions to space said flange from said lid support surface when said lid is placed on said base.
 12. The assembly of claim 1, wherein each of said lugs have a first bottom section having a substantially cylindrical shape and a substantially semi-spherical top section.
 13. The assembly of claim 1, wherein each of said lugs have a first bottom section with a first radial dimension and a second top section with a second radial dimension, where said first radial dimension is greater than said first radial dimension.
 14. The assembly of claim 1, wherein said side wall of said base is substantially annular and where said lugs have a height that is about one-half a height of said side wall.
 15. A container assembly for use in sampling a surface for the presence of microorganisms, comprising: a base having a bottom support surface for supporting a sampling media, a continuous side wall surrounding said bottom support surface and extending away from said bottom support surface to define a height of said continuous side wall, and an odd number of lugs protruding outwardly from an outer surface of said continuous side wall, and a peripheral lid support surface extending outwardly from said continuous side wall; and a lid having a substantially planar surface, a continuous side wall extending from said planar surface and having an inner surface and an inner dimension to be placed on said base and overlie said side wall of said base, said lugs on said side wall of said base having a dimension to contact said inner surface of said side wall of said lid and being spaced apart a distance to deflect said side wall of said lid outwardly at a point of contact with said lugs and to deflect said side wall of said lid inwardly in an area between said lugs.
 16. The container assembly of claim 15, wherein said lugs are uniformly spaced apart to define an area for inward deflection of said lid whereby said side wall of said lid deflects inwardly into said area when said lugs mate with said inner surface of said side wall of said lid.
 17. The container assembly of claim 15, wherein each of said lugs have a contact surface area for contacting said inner surface of said side wall of said lid and where said contact surface area is sufficient to grip said base.
 18. The container assembly of claim 17, wherein said lugs have a dimension to grip said side wall of said lid with a gripping force that is about twice the weight of said base.
 19. The container assembly of claim 15, wherein said side wall of said base includes three of said lugs uniformly spaced apart around said base whereby said lugs define three points of contact with said lid and deflect said side wall of said lid outwardly at said three points of contact to define a resilient gripping force.
 20. The container assembly of claim 15, wherein said side wall of said base includes five of said lugs uniformly spaced apart around said base whereby said lugs define five points of contact and outward deflection of said inner wall of said lid to define a resilient gripping force.
 21. The container assembly of claim 15, wherein said lugs are uniformly spaced around said base and where said lugs have a longitudinal dimension, a width and a convex outer surface having a surface area sufficient to form a friction fit with said lid.
 22. The container assembly of claim 15, wherein said lid is made of polystyrene and where said lugs have a width and a radial dimension sufficient to deflect and deform said side wall of said lid without cracking or breaking said lid.
 23. The container assembly of claim 22, wherein said side wall of said base and said lugs remain dimensionally stable when said lid is placed on said base.
 24. The container assembly of claim 15, wherein said side wall of said lid includes an outwardly extending radial flange, said flange having a plurality of spaced apart protrusions to contact said peripheral lid support on said base when said lid is placed on said base.
 25. The container assembly of claim 15, wherein each of said lugs have a first bottom section contiguous with said lid support surface, a substantially convex outer surface and a top section with a substantially semispherical surface.
 26. The container assembly of claim 15, wherein each of said lugs have a height that is about one-half a height of said side wall of said base. 